This invention relates generally to the field of ophthalmic lenses and, more particularly, to bifocal, varifocal or multifocal intraocular lenses (IOLs).
The human eye in its simplest terms functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of a lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and lens. When age or disease causes the lens to become less transparent, vision deteriorates due to an inadequate image or by the scattered and diminished light which can be transmitted to the retina. This deficiency in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an IOL.
The majority of ophthalmic lenses, including IOLs, currently used are of a monofocal design, (i.e., having one fixed focal length). The focal length of the implanted IOL generally is chosen to optimize distance vision at 3 meters from the patient. Most patients receiving an IOL still require glasses for clear distance and near vision.
Various multifocal ophthalmic lens designs are currently under investigation and these designs generally fall into one of two categories, refractive lenses and diffractive lenses. Refractive lenses are more fully described in U.S. Pat. Nos. 5,147,393, 5,217,489 (Van Noy, et al.), U.S. Pat. No. 5,152,787 (Hamblen), U.S. Pat. No. 4,813,955 (Achatz, et al.), U.S. Pat. Nos. 5,089,024, 5,112,351 (Christie, et al.), U.S. Pat. Nos. 4,769,033, 4,917,68, 5,019,099, 5,074,877, 5,236,452, 5,326,348 (Nordan), 5,192,318, 5,366,500 (Schneider, et al.), U.S. Pat. Nos. 5,139,519, 5,192,317 (Kalb), U.S. Pat. No. 5,158,572 (Neilsen), U.S. Pat. No. 5,507,806 and PCT Publication No. WO 95/31156 (Blake) and U.S. Pat. No. 4.636,211 (Nielsen, et al.), the entire contents of which are incorporated herein by reference. Diffractive lenses use nearly periodic microscopic structures on the lens to diffract light into several directions simultaneously. This is similar to a diffraction grating and the multiple diffraction orders focus the light into various images corresponding to different focal lengths of the lens. Diffractive multifocal contact lenses and IOLs are more fully discussed in U.S. Pat. No. 5,178,636 (Silberman), U.S. Pat. Nos. 4,162,122, 4,210,391, 4,338,005, 4,340,283, 4,995,714, 4,995,715, 4,881,804, 4,881,805, 5,017,000, 5,054,905, 5,056,908, 5,120,120, 5,121,979, 5,121,980, 5,144,483, 5,117,306 (Cohen), U.S. Pat. Nos. 5,076,684, 5,116,111 (Simpson, et al.), U.S. Pat. No. 5,129,718 (Futhey, et al.) and U.S. Pat. Nos. 4,637,697, 4,641,934 and 4,655,565 (Freeman), the entire contents of which are incorporated herein by reference.
While a diffractive IOL may have a number of focal lengths, generally, IOLs with only two focal lengths (far and near) are the most common. As with any simultaneous vision multifocal lens, a defocused image (or images) is superimposed (on the retina) the focused component because of the second lens power, but the defocused image is rarely observed by the user, who concentrates on the image of interest. The defocused image acts as a veiling glare source and thus interferes and degrades the focused image. Under certain circumstances (for example, at night), the pupil diameter of the user can expand to 5 millimeters (mm) or more, and a discrete distant light source (e.g., automobile headlights or street lights) can appear to be surrounded by a "halo" or "rings". A significant component of the halo is caused by the light that is directed to the near image which becomes defocused at the retina. The visibility of the halo is affected by the diameter of the lens region directing light to the near image, the proportion of total energy directed to the near image, and the overall imaging aberrations of the eye.
In U.S. Pat. No. 4,881,805, Cohen suggests that the intensity of light traveling through a diffractive lens can be varied by reducing the echelette depth at the lens periphery, thus reducing glare (column 4, lines 63-68). Cohen further states that the zone boundary radii of the diffractive zones need to obey the formula: EQU R.sub.m =.sqroot.2mwf
where:
w=the wavelength of light PA1 m=integer representing the m.sup.th zone PA1 f=focal length of the 1.sup.st order diffraction
Column 5, lines 17-31.
Cohen's theory states that the glare results from the depth of the steps at the diffractive zone boundaries may be more applicable to contact lenses than intraocular lenses. Contact lenses generally move on the eye and the grooves can become filled with debris. In addition, the additive power of the contact lenses generally is less than that of intraocular lenses, which puts the defocused image more in focus, and also the patient's natural residual accommodation may alter the visibility of glare or halos.
U.S. Pat. Nos. 5,470,932 and 5,662,707 (Jinkerson), the entire contents of which is incorporated herein by reference, discloses the use of yellow dyes in ophthalmic lenses to block or lower the intensity of near UV and blue light (between 300 nanometers and 500 nanometers) that passes through the lens. Near UV and blue light is believed to be hazardous to the retina, and including blue-blocking dyes in the IOL is believed to restore the retinal protection lost when the natural lens is removed. Prior to the present invention, there has been no recognition in the art of using near UV and blue light blocking dyes to reduce the glare and halos that can be associated with multifocal IOLs.
Accordingly, a need continues to exist for a multifocal IOL that minimizes glare or halos.